Recently, in response to social needs or movement with the background of energy and environmental issues, a fuel cell has been noticed as a vehicle drive source and a stationary power source. A fuel cell is classified into various types based on electrolyte types or electrode types, which are represented by an alkali type, a phosphoric acid type, a fused carbonate salt type, a solid electrolyte type and a solid polymer type. Among these, because of operability at low temperature (usually not higher than 100° C.), a proton-exchange membrane fuel cell (PEFC) has been noticed and development and practical applications thereof have recently been progressing as a source of power for a low pollution type automobile (JP-A-2004-79457).
Composition of PEFC, generally, has such structure that a membrane-electrode assembly (MEA) is sandwiched by separators. Generally, MEA has structure laminated with a gas diffusion layer, a cathode catalyst layer, a solid polymer electrolyte membrane, an anode catalyst layer and a gas diffusion layer.
In MEA, the following electrochemical reaction proceeds. First, hydrogen contained in fuel gas supplied to an anode (fuel electrode) side is converted to protons and electrons by oxidation with a catalyst. Then, resultant protons pass through a polymer electrolyte contained in the anode side catalyst layer, and further through a solid polymer electrolyte membrane contacting with the anode side catalyst layer, and reach the cathode (air electrode) side catalyst layer. In addition, electrons generated at the anode side catalyst layer pass through a conductive carrier composing the anode side catalyst layer, and further the gas diffusion layer contacting with the anode side catalyst layer at a different side of the solid polymer electrolyte membrane, the gas separator and an external circuit, and reach the cathode side catalyst layer. Then, protons and electrons reached the cathode side catalyst layer react with oxygen contained in oxidizing agent gas supplied to the cathode side catalyst layer, and generate water. In a fuel cell, electricity can be taken out through the aforementioned electrochemical reaction.
As applications of PEFC, a vehicle drive source or a stationary power source has been studied. To be suitable to such applications, durability for a long period is required. In particular, use as a vehicle drive source requires no lowering of cell characteristics caused by frequent start-stop operation.
In particular, in an electrode catalyst layer containing a catalyst comprising platinum or a platinum alloy, conductive carbon material supporting a catalyst, and a proton conductive polymer electrolyte, repeated start-stop operation easily generates corrosion of the conductive carbon material or degradation of the polymer electrolyte by decomposition, and tends to reduce gas diffusion property and drainage property of the electrode, increase concentration over-voltage and lower cell characteristics.
Therefore, many attempts have been taken to improve corrosion resistance of conductive carbon material in the past. For example, JP-A-05-129023 and JP-A-2005-26174 have disclosed conductive carbon material having improved corrosion resistance by controlling crystallinity of carbon by means of heat treatment.